This cycle repeats after the furnace cools down a bit.
Replaced batteries in programmable thermostat same results.
Tried with lower front panel off same result (make sure enough air getting in)
Checked filters etc, all brand new...

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if you have a FAN ON off the stat set it there to get constant fan then call for heat...want to see if the supply air is causing the hi limit shutdown...as said HI limit might be fatigued how is the air sound in the ON position solid on all the register...filters and squirrel on the main fan section clean??? wall mounted return grill should hold up a sheet of paperif air is moving thru the ducts

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Update:
When I switch the fan on thermo to ON the main blower starts.
I checked vents and seem to have proper air flow, plants moving, cold air return holds a sheet of paper.

When I watch the furnace inside it has a Honeywell Fan switch/High Limit with white pull arm for auto/manual.
This unit has a dial which spins up when flames are on like a temp. gauge. It has 3 set positions 100, 140, 200. When the 140 position triggers, I hear a loud metal pop sound that seems to come from the blower relay? If the thermostat fan setting is set to auto the main blower does not start (when it hits the 140 trigger) or anytime, when thermostat fan is forced ON the main blower runs, but I still hear a loud click sound when it hits the 140 setting on the Honeywell dial. The dial then continues to heat up until the 200 position triggers the furnace the shut down hard (all gas, fans go off).

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That's a fan limit switch, made by Honeywell. Very common in old furnaces.

There's a metal that coils up like a spring on the rod that sits in the furnace heat exchanger chamber. You can pull that out and see if the coil is broken from it's frame but I doubt it is if it's working.

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dk4dk,

It seems like your furnace is overheating, that is why it is shutting off or it can be that the fan limit is defected, but that's rarely the case. Many reasons can cause it to over heat, dirty filter, under size return, small ducts,
clogged evap coil, ect. This problem is not a DIY. The reason is if the furnace is overheating, something is wrong with your airflow. My best advise to you is to
call a qualify technician to come and check for safety. I went on a service call last year, The Homeowner tried to fix it himself by asking online, When I got there, I found the heat exchanger has
a 5 inches crack, I tested with my CO, it was reading 89 PPM!(It still give me a chill when thinking about it) If he continues to run that furnace, who know what will happen to him and his family. I can tell you that if I come to a furnace that is
overheating, and if it''s 10-20 years old, 85% of the time, the heater exchanger cracked. Heat exchanger crack because of inadequate airflow from poor install. SO do yourself a favor and call your local HVAC tech.

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The fan relay was changed out, and the Honeywell fan/limit switch was changed out. Both brand new.

Problem still exists. I checked and cleaned the A/C evap coil (it was spotless), and I cleaned the permanent filter mounted above the main blower (above the blower fins)... It wasn't bad either.

The next part is interesting. I took out the blower assembly with motor placed on floor in front of furnace (still connected elec). Turned off the call for heat and just ran blower on Medium speed (24/7 mode). Then I called for high speed fan using the fan/limit dial and the motor kicked up to high speed. Significant different both audible and air movement. When I mount the fan assembly back into the furnace and perform the same test I don't hear or feel a difference in air speed? Its like only 1 speed works inside the furnace??? I've also had someone put face over a vent and they can't feel a difference when I put high speed on??? Yet both work on floor outside the furnace?

Confused to say the least....
Any thoughts appreciated. Thanks in advance! Great site!

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Quote:

Originally Posted by Mr.HVAC

dk4dk,

It seems like your furnace is overheating, that is why it is shutting off or it can be that the fan limit is defected, but that's rarely the case. Many reasons can cause it to over heat, dirty filter, under size return, small ducts,
clogged evap coil, ect. This problem is not a DIY. The reason is if the furnace is overheating, something is wrong with your airflow. My best advise to you is to
call a qualify technician to come and check for safety. I went on a service call last year, The Homeowner tried to fix it himself by asking online, When I got there, I found the heat exchanger has
a 5 inches crack, I tested with my CO, it was reading 89 PPM!(It still give me a chill when thinking about it) If he continues to run that furnace, who know what will happen to him and his family. I can tell you that if I come to a furnace that is
overheating, and if it''s 10-20 years old, 85% of the time, the heater exchanger cracked. Heat exchanger crack because of inadequate airflow from poor install. SO do yourself a favor and call your local HVAC tech.

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why is the fan off limit setting at 50f bring it up to OFF at 80f,and the ON at 130-140 range and turns as the heat X temp rises? to bring on the fan with a hotter discharge air what temp on the dial do you get hi speed when you turn it.HI speed should be for cooling only whats it doing in that controller?you need lower air speeds during heating LO or Med LO the slower the air the better it heats up..maybe MEDIUM if you have leaking supply ducts

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Interesting! I have a few questions and possible tests to explore..
I am assuming your blower fan is set up to run continuously on low speed and then kick onto high speed through your fan controls? (140 and up)

I've seen blowers (that are dying) that can function without back pressure (out of a furnace) but not do very well inside a furnace.
What gets everything to shut down immediately(especially the main blower that should normally keep going through a limit shut down.)?

I'd be suspicious of the motor.
With the blower fan in place in the furnace, I'd be checking if the high speed leads to the motor are still being powered after the fan control goes above 140. You can disconnect the low speed for a cleaner measurement of the high speed line.
What amp draws are you getting on the motor when it's back in the furnace with the door in place? Can you run your meter test leads through the filter slot?
When your blower was outside of the furnace and able to run adequately on medium and high speeds, was it still electrically grounded to the furnace or not?

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Update:
I installed a new main blower motor today and the problem persists! Low speed continuous, until High speed is called and then it kicks in... No problems there.
This motor is more audible when switching between the 2 speeds. I also removed the burners and got a clear shot inside the heat exchanger. No visible cracks! Looks really good actually (used a piece of mirror)...

Recap: Furnace is overheating. Fan/Limit switch rotates to 200 and shuts off gas. Blowers continue to run and cool down the furnace until the cycle repeats.
Parts changed so far: new Fan/Limit switch, new fan relay, new main blower motor.

I've had 2 HVAC techs come to house, one said it was Fan/Limit switch, other said main blower motor. Both thought heat exchanger was fine...
Neither said they had Co2 sniffer tool with them. They said they could tell by watching flames...
I've removed the standard paper filter, and I've cleaned the evap coil inside (it was spotless before I started)... This furnace is 24 years old and has ran like a top, so existing duct work should be fine...

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Some stray musings on your problem.You seem to have two distinct problems with your furnace that I can't track to a single cause.
Chronic over heating and the fact that something is cutting the power to the fan motor when the furnace is limiting out.

2 minutes is a pretty fast limiting time for a 2 speed fan operation. Many furnaces would not even be hot enough to trigger the higher fan speed within 2 minutes, let alone limit.
The only place I'd be looking for a crack in the exchanger would be in the clamshell that housed the spiral bimetal arm of the fan/limit control. ( excess heat from the gas flames directly being pulled through the crack and past the sensing/activating arm of the fan/limit control by the fan. A good co sniffer is needed here.
Note# A furnace tech today who doesn't carry an accredited and anually re calibrated co/co2 detecter in his truck, can not do his job properly and would be better described as a handy man who works on furnaces.
Other odd causes for fast furnace limiting are having the direction of a squirrel cage facing the wrong way or having the motor running in a backwards direction.

As far as the main fan shutting off when the furnace limits???, It is just not supposed to do that.
I have heard of fan limit controls (like yours) that will cut off the fan if the wires for the fan and the limit circuit are exchanged. When this happens, as soon as the limit is reached the power to the fan is cut off.
I think I would trace back the power line from the blower and test each junction point along that route to see what is shutting off the blower power when the furnace limits out.

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Fans are operating ok, that was some poorly relayed information.

On the Honeywell fan limit switch it has a temperature probe (post with coil) that enters the furnace. Does that probe go inside the heat exchanger or between the fins outside the exchanger?
This clam shell you mention, is that a replaceable part? That sounds like the most likely cause now, and I'm anxious to follow this path!
Thanks!